If you look up the error in the manual, it will give you some description of what it is. Off-hand though, it sounds like maybe some of your blocks are too narrow? Are you using grid-snapping when creating blocks? Or did use CSG subtract (never, ever use this--it leaves all kinds of bad blocks behind)? If you absolutely need a very narrow block in your level, you may want to build it (and some of the surrounding architecture) and convert it to a model for re-import instead. Or else re-think what you're building so you don't need such a narrow block...